Cigarette smoking is the leading cause of preventable death in the United States. Smoking produces over 440,000 deaths each year in this country and generates an estimated $167 billion in annual health-related economic losses. Available methods of smoking assessment (e.g., self-report, portable puff-topography instruments) do not permit the collection of accurate, non-reactive measures of smoking behavior that capture real-time smoking frequency and comprehensive within-cigarette puff topography. The objective of this project is to complete development of a non-invasive wearable system (Personal Automatic Cigarette Tracker - PACT) that does not rely on any form of self-report and does not interfere with the natural smoking behavior of an individual, develop new capabilities for monitoring of smoke exposure, fully validate PACT in unconstrained settings, evaluate if PACT can detect whether the use of portable puff topography devices affect smoking behavior, compare the strength of correlation of metrics of smoke exposure measured by PACT and PTI with commonly used smoke-exposure biomarkers, and determine whether smoke exposure metrics collected in the natural environment are related to heaviness of smoking and smoking location. Methodologically, PACT relies on: 1. Wearable sensors for monitoring of breathing and hand-to-mouth gestures. Miniature sensors integrated into an elastic belt worn under regular clothing monitor the breathing and activity patterns of individuals. 2. Computer algorithms utilizing signal processing and pattern recognition methods for analysis of the sensor signals, detection of patterns uniquely identifying smoking events, and computation of smoke exposure metrics. The validation of the PACT system will be addressed in six Specific Aims: Aim 1. Perform pilot testing of the new generation of the PACT device and collect data for algorithm development. Aim 2. Improve computer algorithms to recognize individual smoke inhalations, develop algorithms for characterization of smoke exposure and investigate algorithms' accuracy as a function of users' anthropometric characteristics (gender, BMI, etc.). Aim 3. Validate PACT in unconstrained settings. Aim 4. Evaluate difference in smoking behavior metrics monitored by puff topography and PACT. Aim 5. Evaluate associations between common smoking biomarkers and smoke exposure metrics from PTI and PACT. Aim 6. Examine the extent to which smoking metrics derived from PACT are related to level of smoking and smoking location. This set of Specific Aims will build on the prototype developed in a previous R21 grant, bring the device and associated data processing methods to maturity, enable practical use of PACT in studies of smoking behavior, and facilitate novel applications allowing for real-time feedback to smokers.